342 DANIEL I. ARNON 



process of CO2 assimilation by isolated chloroplasts followed the same 

 pathway as in algal cells and leaves. This was done by subdividing the 

 chloroplasts into component parts and identifying in them, or isolating 

 from them, the individual enzyme systems that account for the conversion 

 of CO2 to carbohydrate [34-36]. The results have established that in 

 isolated chloroplasts, as in whole cells, the conversion of CO2 to carbo- 

 hydrate proceeds by the same series of dark reactions that are driven by 

 ATP and TPNH2 (TPNH2, not DPNHo, was the reduced pyridine 



Triose 

 phosphate ' 



, Hexose , 

 •^phosphate 



STARCH 



Carbohydrate synthesis by isolated chloroplasts. 



Fig. I . Condensed diagram of the reductive carbohydrate cycle in chloroplasts. 

 The cycle consists of three phases. In the carboxylative phase (I), ribulose-5-phos- 

 phate (Ru-5-P) is phosphorylated to ribulose diphosphate (RuDP) which then 

 accepts a molecule of CO., and is cleaved to 2 molecules of phosphoglyceric acid 

 (PGA); in the reductive phase (II) PGA is reduced to triose phosphate; in the 

 regenerative phase (III) triose phosphate is partly converted into Ru-5-P and partly 

 into hexose phosphate and starch. All the reactions of the cycle occur in the dark. 

 The reactions of the carboxylative and reductive phases are driven by ATP and 

 TPNH., formed in the light. One complete turn of the cycle results in the assimila- 

 tion of I mole of CO2 at the expense of 3 moles of ATP and 2 moles of TPNH2. 



nucleotide formed by illuminated chloroplasts). The general scheme for 

 CO2 assimilation by isolated chloroplasts is summarized in Fig. i. 



The validity of the scheme shown in Fig. i was supported by a physical 

 separation of the light and dark reactions of photosynthesis in chloroplasts 

 [37]. The light phase was carried out first by the complete chloroplast 

 system in the absence of COo and resulted in an evolution of oxygen accom- 

 panied by an accumulation of TPNHo and ATP in the reaction mixture. 

 The green portion of the chloroplasts (grana ; cf. Fig. 2) was then discarded 

 and when COo was next supplied to the remaining non-green portion of the 



